1. Field of the Invention
The present invention relates to an electrostatic discharge (ESD) protection device, and particularly to technologies for preventing breakdown and deformation of a ceramic multilayer substrate caused by, for example, cracking in an ESD protection device that includes discharge electrodes that face each other in a cavity of the ceramic multilayer substrate.
2. Description of the Related Art
ESD is a phenomenon in which strong discharge is generated when a charged conductive body (e.g., human body) comes into contact with or comes sufficiently close to another conductive body (e.g., electronic device). ESD causes damage or malfunctioning of electronic devices. To prevent this, it is necessary to prevent the application of an excessively high voltage generated during discharge to circuits of the electronic devices. ESD protection devices, which are also called surge absorbers, are used for such an application.
An ESD protection device is disposed, for instance, between a signal line and a ground of the circuit. The ESD protection device includes a pair of discharge electrodes that face each other with a space therebetween. Therefore, the ESD protection device has high resistance under normal operation and a signal is not sent to the ground. An excessively high voltage, for example, generated by static electricity through an antenna of a mobile phone or other device causes discharge between the discharge electrodes of the ESD protection device, which leads the static electricity to the ground. Thus, a voltage generated by static electricity is not applied to the circuits disposed downstream from the ESD protection device, which protects the circuits.
For example, an ESD protection device shown in an exploded perspective view of FIG. 5 and a sectional view of FIG. 6 includes a cavity 5 provided in a ceramic multilayer substrate 7 made by laminating insulating ceramic sheets 2. Discharge electrodes 6 that face each other and that are electrically connected to external electrodes 1 are disposed in the cavity 5 that includes discharge gas. When a breakdown voltage is applied between the discharge electrodes 6, discharge is generated between the discharge electrodes 6 in the cavity 5, which leads an excessive voltage to the ground. Consequently, the circuits disposed downstream from the ESD protection device are protected (see, for example, Japanese Unexamined Patent Application Publication No. 2001-43954).
However, there are problems with such an ESD protection device.
In the ESD protection device shown in FIGS. 5 and 6, the responsivity to ESD easily varies due to the variation in the space between the discharge electrodes. Furthermore, although the responsivity to ESD can be adjusted by changing an area of the region between discharge electrodes that face each other, the amount of adjustment is limited due to the size of the product. Therefore, it can be difficult to achieve the desired responsivity to ESD.